A typical IP Multimedia Subsystem (IMS) requires each subscriber to register to an instance of a Serving-Call/Session Control Functions (S-CSCF). In typical implementations, the S-CSCF is statically configured and tied to a piece of hardware, e.g., a server, that stores the registration for the particular subscriber. Once the subscriber is registered to an S-CSCF, all traffic to/from that subscriber, e.g., SIP phone, 3G wireless handset, IP PBX, teleconferencing, or a carrier using the wholesale access of a wholesale carrier, is delivered through that instantiation of the S-CSCF. The result is that even if IMS S-CSCF functions can be supported on multiple servers, the capacity to handle traffic and the reliability of calls completing to/from a particular subscriber is tied to the capacity and reliability of the individual server with which the subscriber is registered. This can be problematic as it requires a high performance/highly reliable server to provide the high quality of service required by Business VOIP (BVOIP) enterprise customers, and to provide the service levels Consumer VOIP (CVOIP) customers are accustomed.
For example, in an enterprise domain, some services have high concentrations of calls to or from a given telephone number. As an example of concentration of calls being place to a particular number, some reality shows have user voting that results in peaks of traffic being generated to the same relatively small set of phone numbers over a short time period. Similarly, there may be a concentration of calls being placed from a given telephone number. For example, a telemarketing company may place a large number of calls that originate from a given telephone number or lines on a PBX capable of VoIP calls. In either example, the traffic can overwhelm the capacity of the few servers handling these phone numbers, even while additional server capacity may exist in the network.
Other events may cause calls to be concentrated on a particular number. For example, a natural disaster may cause calls to be concentrated to a relatively few telephone numbers. Further, a large teleconference may cause calls to be concentrated on a single number.